Engineering High-Performance CIGS Solar Cells: Structural Design and Process Development

The pursuit of sustainable energy resources has led to the exponential development of solar cell technologies, with a focus on achieving high power conversion efficiency (PCE) [1]. Within this landscape, copper indium gallium diselenide (CIGS) is highlighted as being one of the most promising semiconductors owing to low price, high absorption coefficient, tunable band gap, low radiation damage, and long-term stability [2,3]. CIGS is a p-type quaternary compound semiconductor and a solid-solution material belonging to the I–III–VI₂ family. It exhibits a chalcopyrite crystal framework with tetrahedrally coordinated atomic bonding, as schematically illustrated in Figure 1 [4]. The performance of CIGS solar cells is strongly influenced by Ga and In composition, which enables bandgap tuning and efficiency enhancement [5,6]. In particular, gallium content plays a crucial role in improving open-circuit voltage, while numerical modeling approaches provide further insight into device optimization [7,8,9]. The material is characterized by a direct bandgap with a high absorption coefficient of $${10}^{5} \,{\mathrm{c}\mathrm{m}}^{-1}$$ [10], which allows the absorber layer to absorb a significant portion of solar radiation with only 2–4 µm thick film [11]. This thickness is roughly 100 times less than the thickness of a c-Si wafer-based Pv cell, which is in the range of 150–200 µm [12].

The architecture of a state-of-the-art high-performance device is critical for optimizing optoelectronic properties. A typical CIGS-based TFSC is a heterojunction solar cell composed of a conventional multilayer stack consisting of soda-lime glass (SLG), Mo back contact, CIGS absorber, buffer layer, window layer, and top metal contact, as illustrated in Figure 2 [13]. In the substrate configuration, incident light enters the cell through the transparent conducting oxide (TCO) front side, while the back contact is deposited on the supporting substrate, as illustrated in Figure 3 [14]. To reach the laboratory record efficiency of 23.6% achieved in early 2023 [10], structural engineering of the interfaces is paramount. This approach relates to the deposition of n-type buffer layers on the CIGS layer, elemental inter-diffusion at CIGS/buffer layer interfaces, presence of ordered vacancy defects, compositional deviation, and the formation of a concomitant $$p-n$$ junction [15,16,17,18].

Specifically, the implementation of a double profiling structure, known as a notch structure, utilizes higher $$Ga$$ content towards the front and the back of the CIGS absorber layer and low $$Ga$$ content in between them Figure 4 [14]. As illustrated in Figure 5, front-side grading enhances the open-circuit voltage ($${V}_{oc}$$), whereas back-side grading improves the short-circuit current ($${I}_{sc}$$) [14].

Process development remains a primary focus for commercializing high-performance cells, prioritizing stoichiometry, efficiency, high-throughput, and reproducibility [14]. For the preparation of the CIGS film, vacuum-based techniques such as co-evaporation and sputtering are more preferable due to the formation of smooth and pinholes-free morphology with controllable thickness over a large area [10]. The three-stage process (NREL process), Figure 6, which involves depositing In and Ga in the first stage, Cu in the second stage, and finishing with In and Ga to provide a graded profile, has currently given the highest efficiency [11]. Furthermore, doping with alkali metals, such as $$Li$$, $$Na$$, $$K$$, $$Rb$$, and $$Cs$$, can effectively modify the bandgap and optimize the PCE of the absorber layer [1]. In addition to terrestrial applications, these high-performance engineered cells are suitable candidates for space applications due to their high radiation tolerance [19].

The corresponding energy band diagram of the CIGS solar cell is illustrated in Figure 4. The diagram illustrates band alignment, carrier transport mechanisms, and heterojunction behavior within the device.

The energy band diagram provides a clear physical interpretation of band alignment and carrier transport across the device. The heterojunction formed at the CdS/CIGS interface facilitates efficient separation of photogenerated carriers, while band bending enhances carrier collection and minimizes recombination losses. The band alignment between different layers plays a crucial role in determining the overall device performance.

Furthermore, the continuous development of thin-film photovoltaic technologies requires a deeper understanding of material properties and device optimization strategies.

CIGS solar cells represent a high-efficiency thin-film photovoltaic technology, whose optoelectronic properties can be effectively tuned through compositional engineering [6,20,21,22]. Advanced strategies such as absorber thickness optimization and deposition techniques, including magnetron sputtering, have been shown to significantly enhance device performance [5,23,24,25]. Furthermore, the incorporation of novel buffer layers and back surface field (BSF) structures plays a crucial role in improving charge carrier collection and overall energy conversion efficiency [23,26].

This report details the structural design paradigms and fabrication advancements essential for the next generation of power-efficient CIGS solar cells.

Advanced Bandgap Engineering via Double Grading. To mitigate the efficiency constraints inherent in single-junction devices, a primary structural optimization involves the implementation of a double profiling structure of the absorber composition, known as a notch structure Figure 4. This architecture utilizes a higher Ga content towards the front and the back of the CIGS absorber layer and low Ga content in between them Figure 5 [14]. The resulting “V” type band structure boosts light absorption, accelerates charge transport, reduces recombination, and increases $${V}_{oc}$$ [4]. Specifically, front side grading improves $${V}_{oc}$$, while back grading improves $${J}_{sc}$$ Figure 5 [14].

The back surface field (BSF) created by this Ga gradient reflects electrons towards the p–n junction to be finally collected by the n-type electrode, thereby reducing minority carrier recombination at the rear CIGS/Mo interface [14]. This precise modulation of the Ga/In ratio through the absorber depth creates a graded bandgap composition (typically 1.04 eV to 1.7 eV) that provides a better match to the solar spectrum [4].

Interface Engineering and Surface Passivation. A critical solution for reducing open-circuit voltage deficits is the engineering of the absorber/buffer interface. Recombination at the heterogeneous $$CI\left(G\right)S/CdS$$ interface is effectively reduced by the formation of an ordered vacancy compound (OVC or ordered defect compound, ODC), which acts as a buried pn junction [12]. Furthermore, doping with alkali metals—$$Li$$, $$Na$$, $$K$$, $$Rb$$, and $$Cs$$—is proposed to modify the bandgap and optimize the PCE [1].

Heavy alkali metals like $$Rb$$ and $$Cs$$ contribute to phase formation with $$Se$$ and In at the surface, which improves band alignment at the interface and enhances electrical properties [1]. The implementation of alkali doping via post-deposition treatment (PDT) is a primary strategy to enhance the p–n junction interface [1]. For instance, $$K$$ doping facilitates the diffusion of $$Cd$$ into the $$Cu$$-depleted surface, improving the quality of the $$CIGS/CdS$$ interface [28].

Buffer Layer and Window Layer Optimization. To address the parasitic absorption of high-energy photons caused by conventional $$CdS$$, structural optimization favors the use of wide-bandgap, $$Cd$$-free buffer layers. Atomic Layer Deposition (ALD) is identified as a superior technique for producing uniform and conformal films like $$Zn\left(O, S\right)$$, $$ZnMgO$$, and $$ZnSnO$$ [13]. These materials provide a higher bandgap than $$CdS$$, which leads to lower absorption losses [11].

Proper formation of the conduction band offset ($$CBO$$) is essential; a moderate spike (positive $$CBO$$) within the range of 0–0.4 eV at the CIGS/buffer interface is favorable as it does not harm current collection [29,30]. Conversely, a cliff (negative CBO) decreases the barrier at the interface and increases recombination, leading to voltage losses [31]. Window layer optimization involves the use of $$i-ZnO$$ to act as a blocking layer for shunt paths between the front contact and absorber layer [11].

Technological Pathways for Implementation. The most effective fabrication pathway to achieve these optimized structures is the vacuum-based three-stage co-evaporation process (NREL process) Figure 6 [14]. This sequence involves [27]:

Alternatively, sequential processing involving sputtering followed by sulfurization-after-selenization (SAS) provides the highest device performance for large-area production [10]. For flexible applications, low-temperature co-evaporation on polyimide (PI) substrates is essential, often requiring ex situ alkali-PDT to achieve efficiencies exceeding 20% [32].

Theoretical Imperative for Tandem Architectures. Single-junction CIGS photovoltaics, while demonstrating high efficiency, are fundamentally capped by the 33.7% Shockley–Queisser limit [3,32]. This thermodynamic constraint mainly originates from two dominant losses: the transmission of sub-bandgap photons and the thermal relaxation of photogenerated hot carriers, as illustrated in Figure 7a [10]. Reducing the bandgap increases light absorption and enhances short-circuit current density ($${J}_{sc}$$), but the open-circuit voltage ($${V}_{oc}$$) remains constrained by carrier thermalization losses [10]. Tandem configurations overcome this limitation by combining a wide-bandgap top subcell with a narrow-bandgap bottom subcell, thereby broadening solar spectrum utilization, as shown in Figure 7b. This approach provides a pathway to exceed the SQ limit [4]. Theoretical calculations indicate that a two-junction device could boost efficiency up to 46% [10].

Structural Paradigms: Two-Terminal (2T) vs. Four-Terminal (4T) Systems. Realizing these efficiencies requires precise structural integration. Perovskite/CIGS tandem cells hold great promise (Table 1) because the bandgaps of both absorbers can be easily tailored for current matching conditions [19].

Advanced Optical Management and Bandgap Engineering. Optical management is considered a crucial factor for boosting the tandem efficiency [10]. To achieve maximum performance, the subcell bandgaps must be optimized for spectral splitting.

Performance Comparison. The following Table 2 highlights the efficiency benchmarks for single-junction and tandem CIGS-based architectures.

The evolution of CIGS photovoltaics from single-junction films to sophisticated tandem systems represents a significant shift toward the next generation of power-efficient energy resources. Research indicates that the synergy between bandgap tunability (1.04 to 1.7 eV) and interfacial engineering–specifically through Cd-free buffer layers and alkali doping—has already pushed laboratory efficiencies to 23.6%. Transitioning to tandem structures further unlocks the theoretical potential to 46%, with current perovskite/CIGS mechanical stacks already reaching 29.0%. While 2T monolithic designs offer the most streamlined industrial pathway, success remains contingent on meticulous current matching and the optimization of transparent interconnecting layers. Ultimately, the continued refinement of co-evaporation and sequential processing ensures that CIGS remains a cornerstone of flexible, stable, and ultra-high-efficiency solar technology.

The findings of this study highlight the critical role of compositional engineering and interface optimization in achieving high-performance CIGS solar cells. Furthermore, these results provide valuable guidance for the development and large-scale implementation of next-generation photovoltaic systems in both industrial and sustainable energy applications.

During the preparation of this manuscript, the authors used ChatGPT (OpenAI) to assist with language editing and refinement. After using this tool, the authors carefully reviewed and edited the content and take full responsibility for the content of the published article.

Conceptualization, A.K.; Methodology, D.I.; Software, D.I.; Validation, D.I. and A.K.; Formal Analysis, D.I.; Investigation, D.I.; Resources, A.K.; Data Curation, D.I.; Writing—Original Draft Preparation, D.I.; Writing—Review & Editing, D.I. and A.K.; Visualization, D.I.; Supervision, A.K.; Project Administration, A.K.

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No new data were created or analyzed in this study. Data sharing is not applicable.

This research received no external funding.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.